Highly Optically Selective and Thermally Insulating Porous Calcium Silicate Composite SiO₂ Aerogel Coating for Daytime Radiative Cooling

Abstract

Daytime radiative cooling technology offers a lowcarbon, environmentally friendly, and nonpower-consuming approach to realize building energy conservation. It is important to design materials with high solar reflectivity and high infrared emissivity in atmospheric windows. Herein, a porous calcium silicate composite SiO 2 aerogel water-borne coating with strong passive radiative cooling and high thermal insulation properties is proposed, which shows an exceptional solar reflectance of 94%, high sky window emissivity of 96%, and 0.0854 W/m•K thermal conductivity. On the SiO 2 /CaSiO 3 radiative cooling coating (SiO 2 −CS-coating), a strategy is proposed to enhance the atmospheric window emissivity by lattice resonance, which is attributed to the eight-membered ring structure of porous calcium silicate, thereby increasing the atmospheric window emissivity. In the daytime test (solar irradiance 900W/m 2 , ambient temperature 43 °C, wind speed 0.53 m/s, humidity 25%), the temperature inside the box can achieve a cooling temperature of 13 °C lower than that of the environment, which is 30 °C, and the theoretical cooling power is 96 W/m 2 . Compared with the commercial white coating, SiO 2 −CS-coating can save 70 kW•h of electric energy in 1 month, and the energy consumption is reduced by 36%. The work provides a scalable, widely applicable radiative-cooling coating for building comfort, which can greatly reduce indoor temperatures and is suitable for building surfaces.